The invention relates to a silver filled adhesive and more particularly to a single step, low temperature curing silver-filled, polyimide adhesive which is specifically adapted to bond silicon semiconductive devices to substrates.
Existing polyimide die attach adhesives, based on acetylene terminated resins, required a two step cure at high temperature (pre-dry to remove solvent, then final cure to crosslink). Final cure with said existing polyimides has generally required heating over 250.degree. C., which damages some lead frame materials, and which produces very high thermal stresses during cool down. The two step cure process raises cost, slows production, and prohibits use of the adhesive on heat-sensitive parts.
Bismaleimide (BMI) resins have been used extensively as unfilled binder solutions for laminates and adhesives. BMI resins have not hitherto been used in filled paste adhesives due to the inherent instability with many filler solvent combinations. The maleic end groups of BMI resin, the preferred base resin of this invention react much more readily than the acetylene end groups of polyimides used in conventional die attach adhesives. Thus, using bismaleimide, an adhesive has been produced which will yield superior properties when cured at low temperatures (140.degree.-180.degree. C.).
Accordingly, it is a principal object of this invention to provide a low temperature, single step curing silver filled paste.
It is a further object of this invention to provide such a paste which is stable for long periods at or below room temperature.
It is still a further object of this invention to provide such a paste which will give high electrical conductivity, high strength bonds with a minimum of voids and with low thermal stresses after cure.